THE MORPHOLOGY OF THE NARROW-LINE REGION OF MARKARIAN-3

We present the results of Hubble Space Telescope observations of the S eyfert 2 galaxy Mrk 3. Images were taken with the Faint Object Camera and Wide Field Planetary Camera with filters centered on the emission lines [O II] lambda 3727, [O III] lambda 5007, H alpha and H gamma. Th e narrow-line region (NLR) of Mrk 3 has a striking S-shaped morphology extending over more than 2 '' and comprising a large number of resolv ed knots. Its small-scale structure is remarkably well reproduced in a ll the emission lines observed, with a close knot-to-knot corresponden ce. The ratios between [O II] and [O III] and between [O III] and H ga mma emission-line fluxes are constant. The photon budget confirms that the nucleus of Mrk 3 is heavily obscured along the line of sight. Rad io maps of Mrk 3 shows two highly collimated radio jets and the optica l line and radio emissions are very closely associated. We propose a s cenario where the line-emitting gas is compressed by the shocks create d by the passage of the supersonic jet. The increase in the density du e to the shocks causes the line emission to be highly enhanced in the region where this interaction occurs. On each side of the S, the line emission is, on average, displaced in opposite directions with respect to the radio emission as expected from the finite cooling time of the shocked gas which follows the galaxy rotation. The Faint Object Spect rograph spectrum west of the nucleus shows that the line widths are ve ry large, FWHM similar to 1200 km s(-1), and that the [CO III] lines a re clearly double-peaked as would be expected in this interpretation. The range of shock velocity obtained from the cooling length argument is consistent with the measured line widths. The prominent western rad io lobe has a high-excitation emission-line core, surrounded by a diff use halo of relatively lower ionization. We interpret this as a conseq uence of the sweeping up of gas by the expanding radio lobe. The narro w linear ridge of the S-shaped NLR is misaligned with respect to the s ymmetry axis of the larger scale biconical emission. If there is a wid e angle ionization cone, as in NGC 1068, it must be partly filled by g as and its opening angle must be larger than 110 degrees. It appears t hat, at least on the small scale, the dominant factors in determining the NLR morphology are either the gas distribution or the effects of t he radio outflow rather than the geometry of the ionizing field. A sma ll-scale bar (approximate to 0.35 '' x 1 '') of continuum emission is also observed, possibly due to free-free emission from the hot shocked gas or scattered nuclear light.